1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head including a magnetic pole and a heating element, and to a method of manufacturing the same.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head section including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head section including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium. The slider has a medium facing surface that faces the magnetic recording medium. The medium facing surface has an air inflow end (a leading end) and an air outflow end (a trailing end).
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the magnetic recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the aforementioned problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a magnetic recording medium having high coercivity. When writing data, a write magnetic field and heat are applied almost simultaneously to the area of the magnetic recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
The thermally-assisted magnetic recording head includes a magnetic pole for applying a write magnetic field to the magnetic recording medium, and a heating element for applying heat to the magnetic recording medium. Near-field light is typically used as a means for applying heat to the magnetic recording medium. A plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light, is typically used as the heating element. Thermally-assisted magnetic recording heads including such a plasmon generator are disclosed in, for example, U.S. Patent Application Publication Nos. 2011/0058272 A1 and 2011/0096638 A1, and JP-A-2008-10093.
For conventional recording methods using only magnetism, the main factor contributing to the enhancement of linear recording density is a steep gradient of the change in write magnetic field strength in the direction along the tracks or the direction in which the tracks extend. In addition to this, for thermally-assisted magnetic recording, a change in temperature occurring in the magnetic recording medium in the direction along the tracks due to the heat applied to the magnetic recording medium and a change in coercivity occurring in the magnetic recording medium in the direction along the tracks due to the aforementioned temperature change also contribute to the enhancement of linear recording density. This will now be described in detail.
To achieve the enhancement of linear recording density, it is effective to enhance the abruptness of magnetization transition in the magnetic recording medium. For thermally-assisted magnetic recording, the abruptness of magnetization transition in the magnetic recording medium can be expressed by the effective magnetic field gradient dHeff/dx which is defined by Equation (1) below. The greater dHeff/dx, the higher the abruptness of magnetization transition becomes.dHeff/dx=(−dHc/dT)·(dT/dx)+dHh/dx  (1)
In the equation above, −dHc/dT represents the gradient of the change in coercivity of the magnetic recording medium with respect to the change in temperature of the magnetic recording medium. The term “−dHc/dT” takes on a positive value. The term “dT/dx” represents the gradient of the change in temperature of the magnetic recording medium with respect to the change in position in the direction along the tracks of the magnetic recording medium. The term “dHh/dx” represents the gradient of the change in write magnetic field strength at each position with respect to the change in position in the direction along the tracks of the magnetic recording medium. Hereinafter, dT/dx will be referred to as the gradient of temperature with respect to position, while dHh/dx will be referred to as the gradient of write magnetic field strength with respect to position.
For the conventional recording methods using only magnetism, the main factor contributing to the enhancement of linear recording density is dHh/dx in Equation (1). In addition to dHh/dx, for thermally-assisted magnetic recording, the term “(−dHc/dT)·(dT/dx)” also contributes to the enhancement of linear recording density, i.e., the enhancement of the abruptness of magnetization transition, as can be seen from Equation (1).
In thermally-assisted magnetic recording, on a track of the magnetic recording medium there occur a distribution of the write magnetic field strength that peaks at a given first position and a distribution of the temperature of the magnetic recording medium that peaks at a given second position. Hereinafter, the first position will be referred to as the peak write magnetic field point, and the second position will be referred to as the peak heat point. The peak write magnetic field point and the peak heat point are preferably located close to each other. However, since it is not possible to place the magnetic pole and the plasmon generator at the same position, the peak write magnetic field point and the peak heat point are located at positions different from each other. Therefore, the distribution of the write magnetic field strength and the distribution of the temperature partially overlap each other. The distribution of the temperature causes a distribution of the coercivity of the magnetic recording medium to occur on the track.
Here, the side of positions closer to the leading end relative to a reference position will be defined as the leading side, and the side of positions closer to the trailing end relative to the reference position will be defined as the trailing side. The leading side is the rear side in the direction of travel of the magnetic recording medium relative to the slider. The trailing side is the front side in the direction of travel of the magnetic recording medium relative to the slider.
For thermally-assisted magnetic recording, the position on the track at which a magnetization transition takes place is determined as follows. On the track, on the trailing side relative to the peak heat point, the temperature decreases and accordingly the coercivity increases as the distance from the peak heat point increases. The direction of magnetization is not determined when the coercivity is lower than the write magnetic field strength, but is determined when the coercivity is equal to or higher than the write magnetic field strength. Accordingly, on the trailing side relative to the peak heat point, the position of the point of intersection of the distribution curve of the coercivity and the distribution curve of the write magnetic field strength determines the position at which a magnetization transition takes place. Hereinafter, this point of intersection will be referred to as the write point.
Thermally-assisted magnetic recording heads can be configured such that the peak write magnetic field point is located on the leading side relative to the peak heat point, or such that the peak write magnetic field point is located on the trailing side relative to the peak heat point. It has been found that the latter configuration can raise the following problem depending on the positions of the write point and the peak write magnetic field point relative to each other.
In the configuration in which the peak write magnetic field point is located on the trailing side relative to the peak heat point, the write point is also located on the trailing side relative to the peak heat point. In this case, the write point and the peak write magnetic field point can conceivably be in the following first or second positional relationship with each other. The first positional relationship is such that the peak write magnetic field point is located at the same position as the write point or on the leading side relative to the write point. The second positional relationship is such that the peak write magnetic field point is located on the trailing side relative to the write point.
When the write point and the peak write magnetic field point are in the first positional relationship, in a given region on the trailing side relative to the write point, the coercivity increases whereas the write magnetic field strength decreases as the distance from the write point increases, so that the difference between the coercivity and the write magnetic field strength increases with increasing distance from the write point. In this case, no magnetization reversal will occur on the trailing side relative to the write point. When the peak write magnetic field point is located on the leading side relative to the write point, in particular, at the write point both the gradient of temperature with respect to position dT/dx and the gradient of write magnetic field strength with respect to position dHh/dx in Equation (1) take on a negative value to increase the absolute value of the effective magnetic field gradient dHeff/dx, thereby allowing the enhancement of linear recording density.
On the other hand, when the write point and the peak write magnetic field point are in the second positional relationship, in the region from the write point to the peak write magnetic field point located on the trailing side relative thereto, the coercivity increases and the write magnetic field strength also increases as the distance from the write point increases. Thus, in the aforementioned region, the difference between the coercivity and the write magnetic field strength is small and therefore there is a possibility that magnetization reversal can occur due to variations in coercivity or other factors. This may lead to the problems that the magnetization transition width increases to decrease the linear recording density, and erasure of data or erroneous writing may occur on the trailing side relative to the write point.
Thus, in the configuration in which the peak write magnetic field point is located on the trailing side relative to the peak heat point, it is preferable that the write point and the peak write magnetic field point be in the first positional relationship mentioned above. If there is a large distance between the peak write magnetic field point and the peak heat point, however, the write point and the peak write magnetic field point tend to be in the second positional relationship mentioned above.
On the other hand, the enhancement of linear recording density is achievable by steepening the distribution of temperature so as to increase the absolute value of the gradient of temperature with respect to position dT/dx on the trailing side relative to the peak heat point and thereby increase the absolute value of the effective magnetic field gradient dHeff/dx. However, increasing the absolute value of the gradient of temperature with respect to position dT/dx on the trailing side relative to the peak heat point would cause the write point to be closer to the peak heat point. As a result, the write point and the peak write magnetic field point tend to be in the second positional relationship mentioned above.
In the configuration in which the peak write magnetic field point is located on the trailing side relative to the peak heat point, the first positional relationship, which is preferable, can be readily achieved by reducing the distance between the peak write magnetic field point, and the peak heat point. The distance between the peak write magnetic field point and the peak heat point can be reduced by bringing the magnetic pole and the plasmon generator closer to each other. However, restrictions on the arrangement of the magnetic pole and the plasmon generator should impose limitations on this method for reducing the distance between the peak write magnetic field point and the peak heat point.